Information in the form of text and imagery is commonly transmitted among computers within files such as Microsoft Word documents, Microsoft Excel spreadsheets, Microsoft PowerPoint slides, HTML web pages, XML documents and many other types of files that include text and imagery. Typically, a user viewing such files on a display device can freely copy portions of displayed text and imagery by several well-known means. For example, a user can select a portion of text with an input device such as a mouse or keyboard, copy the selected portion of text and paste it into another document, such as the body of an e-mail. For another example, a user can capture the contents of a screen into a clipboard by performing a screen capture, and then insert the contents from the clipboard into another document.
Text within web pages is particularly susceptible to copying. Web browsers displaying HTML pages typically enable a user to view source files for HTML pages being displayed. For example, in the Microsoft Windows operating system running Microsoft Internet Explorer or Netscape Communicator web browsers, a user merely clicks on a right mouse button when the mouse is positioned over an HTML page, and selects “View Source.” The source file for the HTML page is then typically displayed in its entirety within a new window. A user can then readily select any portion of text from the source file, copy it and paste it into another document.
Some applications, such as Adobe's PDF Acrobat, can create non-editable files that can only be viewed within an application that disables the ability to copy selections of text, such as Adobe's PDF Reader. However, a user can capture any portion of a PDF file displayed on a screen by performing a simple screen capture.
Many information services earn their revenues by providing valuable information to clients. Examples of such services include financial services, marketing services, news services and legal services. Moreover, such information is often provided electronically. Using today's technology, a subscriber who receives such electronic information can easily copy it and e-mail it to others, thereby obviating the need for others to subscribe to the service and pay additional subscription fees.
There is thus a pressing need to find a way to prevent text and imagery that is displayed on a computer from being copied without authorization.
U.S. Pat. No. 5,905,505 of Lesk describes an image-based method and system for protecting text displayed on a screen. Lesk operates on a bit-mapped image of the text. Lesk creates two perturbed images, by adding random bits to the bit-mapped image of the text, and rapidly interlaces the two perturbed images. In this way, a user sees the desired image of the text by averaging both perturbed images, but at any given moment only one of the two perturbed images is displayed on the screen. Thus someone copying data from the screen only captures a perturbed image, which is difficult to decipher.
Lesk is difficult to implement in practice, since (1) the random bits have to be generated in such a way that the average of the two perturbed images appears “clean” and legible, whereas each of the individual perturbed images appears “dirty,” (2) Lesk has to be practiced at the level of a video display buffer, (3) for Internet applications, Lesk has to be practiced for each portion of an HTML page being viewed, and (4) it may not be comfortable for a user to view a monitor that is constantly flickering alternating displays. Moreover, it is possible to overcome Lesk by capturing two screens containing both perturbed images, and then averaging them together digitally.
There is thus a need to find a simpler and more practical way to prevent text and imagery displayed on a computer screen from being copied without authorization.